Perhaps I am missing a detail, but (other than ethics) what keeps a Confederate PBEM player from completing a turn, viewing the results of both players actions, then going back and redoing the turn to take advantage of the film? Then once the Southern player is satisfied with the results, they can send the latest files to the opponent, without the Union player ever knowing that the Rebs have a time machine?

I've never used it myself, but doesn't the password prevent this, if being used?

Passwords prevent the South from viewing the Union side of the film replay, but that is not the issue here. What follows is a lengthy explanation, but in short the South can see the results (from a Southern viewpoint) of both sides orders before deciding whether to send the results as is to the North, or simply relaunch the turn (using the starting files) and change the orders to the Rebel units to take advantage of having seen the results of what the North plans to do.

Detailed explanation follows:

A FOF PBEM game turn consists of two phases, the first where both sides give orders to their respective sides, then the replay phase, where the AI resolves both side’s orders and the participants view the results of that.

The problem lies in the mechanics of the PBEM game. The North starts a game, by giving orders to the Yankee units. The AI creates a file (for this example, call it the NTO1 file) with those orders in it, then the player sends the NTO1 file to the Southern player. The Confederate player launches the game using that (NTO1) file, and in turn gives orders to the Rebel forces.

At this point, the AI resolves the orders from both sides and generates two files. The first is the file the Union uses to give orders for the next turn (call it NTO2). The second file is a film replay of the previous turn (call it Replay1). Both files are under the control of the Southern player. What the Rebel player is supposed to do is send a copy of both files to the Federal player, then take a look at the replay file to see what just happened in the war.

What could happen is that instead of sending both files on, the South could view the replay file, and then decide if the results are what that player wants. If they are, then the Confederate player sends both NTO2 and Replay1 on to the opposing player. If (as an example) however, the Southern player sees that the North moved a unit in an unexpected direction, the South could delete both files (NTO2 and Replay1) and relaunch the game using the NTO1 file.

At this point, the South could change their orders to take advantage of the knowledge of what the Union planned. Furthermore, the Rebel player could look at the results of that action before deciding whether to send the new files on, or try yet another run through.

So even though passwords prevent the South from viewing the Northern version of the replay file, it is not a complete defense from cheating by the Confederate player.

I don't reckon there is anything. We didn't build a lot of anti-cheating mechanisms into the game -- there are far more egregious ways to cheat than that if you're willing to spend a small amount of time cracking the save game file, for instance -- mainly because I never really thought that the sort of people who play these sorts of games would be the sort of people who cheat. Although people sometimes speculate about problems associated with cheating, in the seven years we've had these games out, nobody has once that I can recollect complained about an actual case of cheating.

I suppose that “...I never really thought that the sort of people who play these sorts of games would be the sort of people who cheat.” is one approach to simulation design. The idea has an innocent charm to it, and definitely saves on development expenses.

This assumption leads to a question. If the application was created under the caveat that people who play the game don’t cheat, then why go to the trouble of providing a password mechanism?